Magnetic recording media in which a magnetic layer comprising a dispersion of a ferromagnetic powder comprising a circular crystals of .gamma.-Fe.sub.2 O.sub.3, cobalt containing magnetic iron oxide, CrO.sub.2 etc. in a binding agent (binder) is provided on a non magnetic support are generally used as magnetic recording media (also referred to below as magnetic tapes) for audio, video and computer applications etc.
However, more recently a demand has arises for higher recording densities on magnetic recording media and the use of ferromagnetic alloy powders of which the main components are metals such as iron, nickel and cobalt etc. in place of the ferromagnetic powders used conventionally has a become generalized. The ferromagnetic alloy powders enable high density recording to be achieved since they have a high coercive force (Hc) and a high residual flux density (Br), and they are ideal as ferromagnetic materials for magnetic recording media in cases where high density recording is required.
In the case of video tapes in particular. it is necessary to achieve very high density recording in view of the methods such as the shortening of the recording wavelength and the narrowing of the track width which have been used. Video tapes in which ferromagnetic alloy powders are used in place of the conventional iron oxide based ferromagnetic powders can be used for this purpose.
When a ferromagnetic alloy powder is used, higher density recording can be achieved by rendering the surface of the magnetic layer smoother, and it is known that the electromagnetic conversion characteristics of magnetic recording media can be improved in this way.
However, if the surface of the magnetic layer is made smoother, then the coefficient of friction for the contact between the magnetic layer and the apparatus in the video tape running system is increased and the magnetic layer of the magnetic recording medium is liable to be damaged in a short period of time in use or it may tend to peel away in use. With video tapes in particular, the magnetic medium runs at high speed while in contact with the video head and so ferromagnetic powder is easily dropped from the magnetic layer and this is a cause of magnetic head blockage (clogging). Moreover, the running conditions are especially severe in the still mode. Hence, it is desirable that the running durability of the magnetic layers of video tapes should be improved.
Measures for improving the running durability of magnetic layers which have been suggested in the past have included the addition of abrasive agents (hard particles) such as corundum, silicon carbide, chromium oxide etc. to the magnetic layer, but it is difficult to realize an improved effect with the addition of abrasive agents to a magnetic layer with the intention of improving the running durability of the magnetic layer unless the abrasive agent is added in large quantities. However, magnetic layers to which large amount of abrasive agents have been added cause marked wearing of magnetic heads and it is contrary to the effect to the magnetic layer be rendered smoother so as to the electromagnetic conversion characteristics be improved. Thus, this cannot be said to be a preferred method.
Furthermore, fatty acids and esters of fatty acid and aliphatic alcohol have been added to magnetic layers as lubricants in order to reduce the coefficient of friction.
However, with the recent advent of the portable video tape recorder it is to be expected that video tapes will be used under more severe conditions at low temperatures and under conditions of high temperature and humidity. Hence, the video tapes must be so stable that there is no change in their running durability under the various conditions which can be envisaged. However, satisfactory running durability cannot be obtained with the conventional lubricants.
Rapid progress is being made with the miniaturization of recording media by compressing the recording wave-length and track width, especially in the case of video tapes and floppy disks, and ferromagnetic alloy powders are more useful than the conventional iron oxide based ferromagnetic powders. It is possible to obtain superior electromagnetic conversion characteristics by greatly reducing the size of the magnetic particles in this way, but it is difficult to obtain good running durability at the same time and improvement is desirable.
The use of diethylene glycol ester derivatives as lubricants provides one method of ameliorating these problems, and the use of combinations with polyurethane based resins, vinyl chloride based copolymers and epoxy based resins etc. as binding agents has been suggested (JP-A-59-227030, JP-B-57-46128). Furthermore, the use of combinations of oligoalkylene glycol ester derivatives and binding agents which have polar groups has also been suggested (JP-A 63-103419) (the term "JP-A" used herein means an "unexamined published Japanese patent application", and the term "JP-B" used herein means an "examined Japanese patent publication").
Some improvement in respect of the occurrence of head blocking under conditions of high humidity and in respect of the rise in the .mu. value (the coefficient of friction) which arises as a result of repeated running under room temperature conditions can be achieved by means such as those indicated above. However, even with such improvements, the still durability remains inadequate and they, cannot be said to provide a satisfactory performance, especially under low temperature conditions.